Limbs long, bent at elbows, knees; fingers, toes free; face more human-like; tail disappears; umbilical herniation persists to end of third month

epithelial lining of the tympanic cavity and auditory tube originate in the endodermal germ layer

Craniocaudal patterning of the embryonic axis is controlled by homeobox genes. These genes, conserved from Drosophila, are arranged in four clusters, HOXA, HOXB, HOXC, and HOXD, on four different chromosomes. Genes toward the 3' end of the chromosome control development of more cranial structures; those more toward the 5' end regulate differentiation of more posterior structures. Together, they regulate patterning of the hindbrain and axis of the embryo (Fig. 5.22).

As a result of formation of organ systems and rapid growth of the central nervous system, the initial flat embryonic disc begins to fold cephalocau-dally, establishing the head and tail folds. The disc also folds transversely (lateral folds), establishing the rounded body form. Connection with the yolk sac and placenta is maintained through the vitelline duct and umbilical cord, respectively.

Problems to Solve

1. Why are the third to eighth weeks of embryogenesis so important for normal development and the most sensitive for induction of structural defects?

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